This invention relates in general to digital television systems and in particular to a black expander for a digital video signal in a digital television system.
In the above-described copending application, a digital black expander expands digital video signals that are below an arbitrary breakpoint. That invention, in its preferred embodiment, initially develops a moving average of samples of horizontal video and then samples the horizontal moving average at a field rate to develop field rate samples. The breakpoint is established by adding a constant to the moving average of field rate samples. On the contrary, the present invention method and apparatus ascertain the dynamic range of the digital video signal for each field of video and uses the dynamic range to determine black signal level expansion. In the absence of a dynamic range for the video signal, no black expansion is permitted even though the video signal is below the expansion tilt point. With the system of the invention, black expansion is not performed except under conditions where doing so will provide a benefit to the viewer. The premise is that if the video signal has no dynamic range, or only limited dynamic range, there is no need for black expansion. Indeed, black expansion under uniform signal conditions may be detrimental to the video display. The high frequencies in the luminance signal are removed prior to processing to eliminate rapid transitions which could create annoying flicker in the displayed video.